In general 1 to 2lbs of fruit per gallon of beer is fine however strawberry is a very mild fruit flavor. I suggest bumping it up to 2lbs per gallon in secondary and also NOT to use the 300 yeast because the yeast flavor will over power the fruit flavor. I suggest using something like wyeast 1010 or even 1056/ safe ale 05 to help the fruit shine through. Also try to get fresh fruit and freeze it for a fews day prior and let thaw prior to adding followed by mashing it up with a sanitized masher to get more surface area for flavor. The fresh really adds a little bit something extra. O also mash for 90 min to help with conversion.

i would say increase it to 10lbs. Strawberry is a Very very very subtle flavor and the hefe yeast will overpower it. I have tried it and yeast dominates but hell ya never know it may be the best beer you ever made its strictly personal choice. Would love to know the results.

Its going to depend on what strawberries you use. I grow sarian strawberries that have about 50X more flavor to them then the tasteless red "water bags" that you can buy in a store. Remember that the strawberries sold in a grocery store are genetically modified to last longer after being picked than what a "real" strawberry.

I've never made a strawberry beer so I can't suggest an amount to use. If I were going to make one, I'd get the strawberries from a local farm where you pick them yourself.